Source note · SIP ALG problems on firewalls and routers
An ALG can be detected by capturing the same SIP message on both sides of the NAT device. If Via, Contact, Record-Route or the SDP c= and m= values differ between the two captures, the device has rewritten the SIP payload, beyond translating the IP and UDP headers.
inferred · Checked 2026-09-25
- Vendor
- multi-vendor
- Product
- SIP through NAT
- Subsystem
- troubleshooting
- Deployment
- on-premises, multi-tenant, hybrid
- Region
- not restricted
- Release range
- current as of 2026-09-25
- Status
- inferred
- Checked
- 2026-09-25
Sources
- SIP ALG | Junos OS — Juniper Networks · tier 2 current vendor documentation · section listing rewritten headers and SDP fields
Cite this note
APA
WarmTransfer. (2026, September 25). SIP ALG problems on firewalls and routers: source note sip-alg-problems-capture-detection-method. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-alg-problems-capture-detection-method
BibTeX
@misc{warmtransfer-claim-sip-alg-problems-capture-detection-method,
title = {SIP ALG problems on firewalls and routers: source note sip-alg-problems-capture-detection-method},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-alg-problems-capture-detection-method},
note = {Source note sip-alg-problems-capture-detection-method, checked 2026-09-25}
}